Thiophenes are used, for example, for the preparation of conductive polymers. Poly(3,4-alkylenedioxythiophenes) such as are described, for example, in EP-A 339 340, are of particular interest in this context. These compounds are distinguished by particular properties, such as high conductivity, high transparency and outstanding long-term stability. They have therefore found increasing use in industry as organic conductive polymers. Thus e.g. through-plating of printed circuit boards, antistatic treatment of photographic films and use as an electrode or solid electrolyte in solid electrolyte capacitors are described as important fields of use.
An important prerequisite in the preparation of organic conductive polymers is high purity of the starting substances needed for their preparation. Impurities contained in the starting substance can adversely influence the polymerization in that the polymerization does not take place, or takes place only very slowly or incompletely, or is accelerated to an uncontrolled extent. The processing time of these monomers can consequently drop drastically, so that these can no longer be employed in the processing processes.
In addition, the properties of the resulting polymers may also be adversely influenced in that the impurities, for example, adversely change the intrinsic colour of the resulting polymer and as a result the transparency, which is essential for the use of the polymers e.g. as transparent conductive or antistatic coatings, is impaired.
Impurities which are also capable of polymerization can be co-incorporated into the polymer and thereby significantly lower the conductivity thereof. Further adverse effects of impurities can be that the order of the conductive layers may be lowered by impurities, whereby poorer conductivities result, that impurities become concentrated on the surface of the polymer after the polymerization and undesirable transition resistances thereby result, so that the function of the conductive layer is restricted, or that the long-term stability of the conductive polymers is adversely influenced in that the impurities, for example, initiate reaction of the conductive polymer with oxygen and thus significantly impair the properties of the polymer.
The starting substances which are needed for the preparation of organic conductive polymers and are as a rule prepared from raw materials by chemical reactions, are therefore purified before their use.
A number of purification operations which are in principle suitable for purification of the monomers for the polymerization to give organic conductive polymers are known to the expert. Such purification methods are, for example, distillation, sublimation, extraction, crystallization, chromatography and adsorption. These purification methods have been known to the expert for a long time and are described in the usual textbooks.
Thiophenes which are liquid at room temperature and are suitable for the preparation of electrically conductive polymers are of particular importance because of their easy processability in the liquid form. For the purification of these thiophenes the expert has available the purification methods which can be used on liquid substances, preferably distillation, which is also carried out on a large industrial scale, extraction and chromatography.
Distillative purification of thiophenes as monomers for use for the preparation of electrically conductive polymers is known, for example, from EP-A 1 142 888. The doctrine of EP-A 1 142 888 is that the number and amount of by-products can be reduced by optimized reaction conditions and e.g. 3,4-ethylenedioxythiophene is obtainable in a purity of up to 97.7%. However, the doctrine of EP-A 1 142 888 furthermore is that for further purification an additional extraction is necessary in order to remove water-soluble by-products and to achieve a purity of more than 99%. 3,4-Dimethoxythiophene predominantly occurs as a secondary component, i.e. impurity, in this synthesis of 3,4-ethyleniedioxythiophene.
Furthermore, separating off of compounds by distillation is only possible if the components to be separated differ significantly, i.e. by more than 1° C., in their boiling points. The less the boiling points differ, the greater the expenditure on apparatus for the separation, so that such separations are no longer to be carried out economically. Since substituted thiophenes, such as, for example, alkylenedioxythiophenes, are preferably distilled under reduced pressure, the difference in the boiling points is reduced further, which further increases the expenditure on separation.
The purification of 3,4-allylenedioxythiophenes, in particular of 3,4-ethylenedioxythiophene, which are contaminated with 3,4-dimethoxythiophene represents a particular difficulty. Thus, for example, 3,4-dimethoxythiophene produced during the synthesis of 3,4-ethylenedioxythiophene can be separated off only with a high expenditure because of the molecular weight differing by only two units and the very similar structure, which makes purification via distillation no longer economical beyond a certain degree of purity. 3,4-Dimethoxythiophene as an impurity has the disadvantage, however, that it is co-incorporated into the polymer during polymerization and can thus adversely influence properties of the polymer, such as, for example, the conductivity.
Chromatographic purification of thiophenes as monomers for use for the preparation of electrically conductive polymers is also known. WO-A 02/79295 describes the preparation of liquid and solid chiral alkylenedioxythiophenes and mentions in examples the purification by chromatography on silicon dioxide. The compounds prepared according to WO-A 02/79295 have purities of up to 99.7% after purification. However, chromatographic separation also has disadvantages. Thus, large amount of solvents are needed to carry it out, since the compounds to be separated must be in a very dilute form in order to achieve the desired separation effect. Furthermore, the chromatographic separation cannot be operated continuously with the aid of simple apparatuses, so that in each case only small amounts of the desired purified thiophene are obtained. A continuous separation of large amounts would therefore be associated with an extremely high expenditure on apparatus, so that such a purification of thiophenes can no longer be carried out economically.
Conventional recrystallization in which thiophenes which are solid at room temperature are dissolved at elevated temperature, usually under reflux of the solvent, and are then crystallized out again by cooling is also known for the purification of thiophenes as monomers for use for the preparation of electrically conductive polymers and is described in WO-A 02/79295, but is limited to thiophenes which are solid at room tm.
A particular form of crystallization can also be used for the crystallization of liquid thiophenes. This specific form of crystallization, melt crystallization, is described, for example, in N. Wynn, Chem. Engineering (1986), 93(8), 26-27 and in J. Ulrich and H. C. Bülau, Editor(s): Myerson, Allan S. “Handbook of Industrial Crystallization (2nd Edition)” (2002), 161-179. Melt crystallization is substantially based on cooling a liquid substance until a melt is formed, from which only the substance to be purified crystallizes out. After crystallization, the mother liquid, which in the ideal case contains all the impurities, is separated off. Where appropriate, the crystallized substance is heated gently so that impurities adhering to the product can be removed together with some of the substance which is then melting. However, this process is limited to substances or substance mixtures which contain relatively large amounts of impurities which can be separated off in liquid form. Small amounts of impurities can be removed only uneconomically via this process, since large amounts of the desired compound have to be separated off at the same time in order to wash out the small amount of impurity. Moreover, melt crystallization is critical in respect of the temperature programme and therefore expensive on apparatus.